1. Field of the Invention
The present invention relates to a device for controlling a running behavior of a vehicle such as an automobile, and more particularly, to a device for controlling a turn running behavior of such a vehicle for a higher stability in a forward turn running as well as in a rearward turn running.
2. Description of the Prior Art
In the art of running behavior control of four-wheeled vehicles, an oversteer control (anti-spin control) and an understeer control (anti-driftout control) are well known. In the conventional turn behavior control devices, an oversteering condition is generally judged by an oversteer index based upon the slip angle of the vehicle body increasing beyond a threshold value in a direction opposite to a turning direction, and when it occurs, a front wheel serving at the outside of a turn is braked to generate an anti-spin moment in the vehicle around the braked front outside wheel, with or without a simultaneous reduction control of engine torque. When the front outside wheel is braked to control the oversteering, the cornering force of the braked front outside wheel decreases, so that the front outside wheel might slip to the outside of the turn. Such a front outside slip of the vehicle contributes to the oversteer control. On the other hand, an understeering condition is generally judged by an understeer index based upon a deviation of the actual yaw rate of the vehicle from a theoretical yaw rate calculated based upon the steering angle and the vehicle speed, and when it occurs, a rear wheel or wheels are braked to decelerate the vehicle, with a particular effect that when a rear wheel serving at the inside of a turn is braked, a turn assist yaw moment is generated in the vehicle around the braked rear inside wheel. A simultaneous reduction control of engine torque may also be made. When the rear wheel or wheels are braked to control the understeering, the cornering force of the braked rear wheels decreases, so that the rear wheels might slip to the outside of the turn. Such a rear outside slip of the vehicle contributes to the understeer control.
When a vehicle equipped with such a conventional turn behavior control device is put into an oversteering condition in a rearward turn running, its intrinsic front wheel (virtually rear wheel) serving at the outside of the turn will be braked, when the control device still operates. The braking of the virtually rear outside wheel generates an anti-spin moment in the vehicle around the braked virtually rear outside wheel, also to be effective as an oversteer control. However, the braked virtually rear outside wheel is liable to slip to the outside of the turn, and when it occurs, it cancels the oversteer control.
On the other hand, when a vehicle equipped with such a conventional turn behavior control device is put into an understeering condition in a rearward turn running, its intrinsic rear wheel (virtually front wheel) serving at the inside of the turn may be braked. The braking of the virtually front inside wheel generates an anti-driftout moment in the vehicle around the braked virtually front inside wheel, also to be effective as an understeer control. However, the braked virtually front inside wheel is liable to slip to the outside of the turn, and when it occurs, it cancels the understeer control.
In view of the above-mentioned drawback in the operation of the conventional turn behavior control in a rearward drive, it has been proposed by Japanese Patent Laid-open Publication 7-11645 to adapt such a turn behavior control to a rearward drive of a vehicle with such a modification that a braking for an oversteer control in a rearward drive is applied to a virtually front (intrinsically rear) wheel serving at the outside of the turn, while a braking for an understeer control in a rearward drive is applied to a virtually rear (intrinsically front) wheel serving at the inside of the turn.
Apart from the above matter concerned with the forward/rearward drive, with respect to the rearward turn running performance of the four-wheeled vehicles, it is noted that the vehicles are generally highly liable to deviate from a running course intended by the driver, so that the stability of the rearward turn drive of the vehicle is generally very low. An essential cause for such a turn trace instability in the rearward drive would be an inexperience of most drivers to the rearward drive, but another essential cause is considered to reside in the fact that, in the rearward drive, the point of contact of each of a pair of steered wheels with the ground surface is located ahead of the point of the steering axis of each of the pair of steered wheels traversing the ground surface along the direction of running movement of the vehicle, so that, when the pair of steered wheels are non-driven wheels, the force applied to each of the pair of steered wheels from the ground surface has an effect of augmenting a steering angle input thereto by the steering system.